Getting extra mileage from the Stirling engine

US firm CoolEnergy hopes it can help households turn excess heat into sustainable power, by getting a Stirling engine to work at lower temperatures.

The basic principle behind the Stirling engine – harnessing heat to make a fluid expand and push a piston – could have much more mileage in it if it can be run effectively with less initial heat. A smart company claims to have cracked it well enough to work from the waste heat of a conventional power station smokestack. CoolEnergy they are called, and they’ve got backing from the US Department of Energy and the National Science Foundation.

Hitherto, Stirling engines have typically needed temperatures of 1200 degrees and over to get their pistons moving properly, using the pressure created by the expansion of a fluid when it vaporises in a confined space. CoolEnergy’s innovation, marketed as ‘SolarHeart’, depends on the use of heat exchanger pipes and a nitrogen-based fluid that will do the business at ‘just’ 200-500 degrees. It potentially widens the technology’s applications enormously – putting it within a temperature range that’s satisfied by a lot of industrial exhaust.

CoolEnergy is targeting military and remote settings, where fuel is both costly and hard to come by, and industrial facilities that generate vast amounts of waste heat, in both gas and cooling fluids. The company says the installation could pay for itself within two years.

The technology can also be adapted for use in the home, by combining the engine with a solar thermal system. This way, any heat that isn’t required by the host building can be converted to power. The company claims its integrated SolarFlow System provides 80% of heat, 100% of hot water and 60% of electricity in an average size US home.

The efficiency gains are promising. But, at a time when the US coal industry is pulling out all the stops to clean up its act and stay competitive with ‘unconventional’ shale gas, will this mainly be used to delay the death of that dirty dinosaur? – Roger East

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